The present invention relates to a phase-controlled power switching circuit wherein power from an alternating current (AC) source may be applied to an electrical load in a controllable manner. The invention more particularly relates to a power switching circuit in which power level control is achieved through switching techniques involving phase-control.
Various types of electrical devices are subject to damage upon abrupt application of power to the devices. Such abrupt powering of a load may induce a high inrush or surge current in the load that may create a physical disturbance in the load. For example, the surge current in an incandescent lamp, which is large due to low initial filament resistance, may reduce filament life by causing a mechanical shock to the filament. Such shock arises due to interaction between the ambient magnetic field and the magnetic field generated by the filament current.
Prior art phase-control systems include circuits employing unijunction transistors, vacuum tubes, or thyratrons. Such circuits have proven capable of supplying power to a load in a controlled manner and have reduced or eliminated surge problems. However, these circuits tended to be complex or physically cumbersome, especially the tube circuits.
Prior art phase-control systems using microprocessors are capable of achieving a desired, gradual rise in power supplied to an electrical load. The problem with using a microprocessor is that such a device is costly. A microprocessor is typically much more sophisticated than is required for phase control purposes. Accordingly, the full capabilities of a microprocessor are underutilized and represent a partially wasted expense.
Accordingly, it would be desirable to provide a phase-control power switching circuit that avoids the unnecessary expense of a microprocessor-based system.